Oxidation of carbon in the presence of catalysts based on cesium lanthanum vanadate

Article abstract of DOI:10.1023/B:RJAC.0000044160.75371.85

Some kinetic aspects of the catalytic oxidation of carbon with atmospheric oxygen and nitrogen oxide NO in the presence of catalysts based of cesium lanthanum vanadate were studied. The influence exerted by a promoter, cesium sulfate, and its content on the course of catalytic reactions was analyzed.

Full text of DOI:10.1023/B:RJAC.0000044160.75371.85

Russian Journal of Applied Chemistry, Vol. 77, No. 7, 2004, pp. 1121 1129. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 7,
2004, pp. 1136 1143.
Original Russian Text Copyright
2004 by Ostroushko, Makarov, Minyaev.
CATALYSIS
Oxidation of Carbon in the Presence of Catalysts Based
on Cesium Lanthanum Vanadate
A. A. Ostroushko, A. M. Makarov, and V. I. Minyaev
Research Institute of Physics and Applied Mathematics, Ural State University, Yekaterinburg, Russia
Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences, Yekaterinburg, Russia
Received May 6, 2003; in final form, February 2004
Abstract Some kinetic aspects of the catalytic oxidation of carbon with atmospheric oxygen and nitrogen
oxide NO in the presence of catalysts based of cesium lanthanum vanadate were studied. The influence
exerted by a promoter, cesium sulfate, and its content on the course of catalytic reactions was analyzed.
Recently, the interest of researchers has been at-
tracted by catalytic oxidation of carbonaceous sub-
stances (carbon black) with oxygen and with nitrogen
oxides. These reactions are of practical importance for
treatment of effluent gases in thermal power engineer-
ing, metallurgy, building, and at garbage incineration
plants. The interest is due to the fact that fume gases
contain, in addition to other toxic components, carbon
black, nitrogen oxides, and residual oxygen. These
reactions can be used to neutralize exhausts of diesel
engines and to decrease the exhaust smoke emission.
Discharges of carbon black are dangerous because of
the carcinogenic compounds it contains. Deposits of
carbonaceous substances impair the efficiency of
thermocatalytic neutralizers. There exist complex
oxide catalysts that markedly accelerate the oxidation
of carbon black with the residual oxygen, and some of
these can be used to oxidize carbonaceous substances
via reduction of nitrogen oxides to molecular nitrogen.
increases in going from lithium to cesium. A similar
behavior has been observed for carbon oxidation with
nitrogen oxide N O [9].
2
Thermocatalytic devices for neutralization of waste
gases [10], working in the continuous or batch mode,
should ensure a virtually complete burning-out of the
carbon black absorbed. Carbon black substances of
different origins have different structure and composi-
tion [11]. However, finer structures are stronger than
more complex ones, and, therefore, deposits of diesel
carbon black are mainly composed of formations not
larger than 5 10 m in size. The particles may contain
incompletely burnt oils, mineral components of addi-
tives to combustive-lubricating materials, products
formed in wear of the piston and cylinder, etc. [10].
Impurities of this kind diminish the effective oxida-
tion surface of carbon black and hinder access of
oxidizing agents into pores, so that the particles are
only oxidized on their surface.
Carbon can be oxidized in the presence of solid or
partly molten catalysts [1 7], with complex com-
pounds of vanadium distinguished by their high
catalytic activity. The activity of, e.g., lanthanum
vanadate increases dramatically when alkali metal ions
are incorporated into its structure, with the catalytic
activity the higher, the larger the atomic mass of a
metal [2, 3]. In addition, the promoting effect is
exhibited by alkali metal salts and, in particular,
Cs SO introduced into the catalyst composition.
The mechanism of catalysis of the reactions men-
tioned above is rather complicated; to similar types of
interactions can be referred processes occurring in
catalytic gasification of coal and other carbonaceous
substances [8]. Steam oxygen gasification is catalyzed
by compounds of alkali metals, e.g., by their carbo-
nates. The activity of alkali metal compounds also
The kinetic parameters of combustion of various
carbonaceous substances strongly differ [12]. For
dust-like particles of various coals, the overall activa-
tion energy E of combustion in air varies from 75 to
1
170 kJ mol . Very strongly (by several orders of
magnitude) differ the pre-exponential factors K in the
0
Arrhenius equation, obtained for these reactions.
Determining the kinetic parameters is experimentally
difficult. The individual specific features of materials
lead to a wide scatter of points in experimental depen-
dences. However, despite this circumstance, empirical
2
4
equations relating E and K have been derived. To
0
higher activation energies correspond greater values of
K . There exists an opinion that K is proportional to
0
0
the number of active centers on the carbon surface
[12]. As their number grows, the energy of each center
1070-4272/04/7707-1121 2004 MAIK Nauka/Interperiodica

1122
OSTROUSHKO et al.
decreases, which leads to an increase in the activation
energy. It has been suggested to use as a parameter
describing the process the specific surface rate of reac-
promoter, cesium sulfate Cs SO , an aqueous alco-
2 4
holic solution of a technical-grade reagent was pre-
pared, and the catalyst powder was impregnated with
a prescribed volume of this solution and then dried
at 90 C. Other samples were also prepared by
pyrolysis of polymer salt compositions from the
appropriate salts [13, 17].
C
tion with carbon, K [12]. This quantity depends on
S
whether the process is kinetically or diffusion-con-
trolled. It is related to the concentration of O in the
2
gas medium, relative amounts of the forming oxides
2
1
CO and CO , etc. It is commonly believed that, for
2
The catalyst (S = 8 15 m g ) was ground in
sp
carbonaceous particles of size not exceeding 10 m,
reactions at their internal surfaces at low temperatures
are not important, and the overall combustion rate is
determined by the kinetics of the chemical reaction
an agate mortar and mixed in a mass ratio of 4 : 1 with
a preliminarily ground activated carbon of BAU brand
2
1
[18] (S
100 m g , ash content 4 wt %). The ash
sp
contained oxides of aluminum, iron, silicon, and other
metals (microimpurities). The catalytic activity in
carbon oxidation by atmospheric oxygen was deter-
mined gravimetrically in an open reactor, with the
layer of a mixture of a powdered catalyst with carbon
placed in ceramic boats. Oxidation of carbon with
nitrogen oxide NO was studied in a similar manner in
a flow-through quartz reactor through which NO ob-
tained in a reaction of a concentrated solution of
NaNO or KNO and a solution of FeSO acidified
between O and carbon. If carbon combustion occurs
2
within a bed, the key importance is acquired by diffu-
sion processes, and E decreases severalfold [12].
For the properties of samples in modeling of reac-
tions with gases to be reproducible, a certain type of a
carbonaceous material is chosen [9, 12], e.g., activated
carbon. In this study, experiments were performed
with activated carbon ground to 10 m, which makes
the samples similar in particle size and specific sur-
face area of particles to carbon black. The presence of
mineral substances in the carbon makes the experi-
mental conditions close to reality. An attempt was
made to carry out a comparative analysis of kinetic
features of the process of carbon oxidation by at-
mospheric oxygen and nitrogen oxide NO in the pres-
ence of a cesium lanthanum vanadate catalyst with
a monazite structure, promoted with various amounts
of Cs SO .
2
2
4
with hydrochloric acid (all reagents of chemically
1
pure grade) was passed at a flow rate of 30 ml min .
The first solution was added dropwise to the second.
To determine the optimal composition of the cata-
lyst, Cs La VO (x = 0 to 0.5; monazite struc-
4 y
x
1
x
ture), and M La Cu
V
O
(M = K, Cs),
0.1 1.9 0.95 0.05
4
y
which belong to the K NiF structural type, were syn-
2
4
thesized as single-phase samples. The introduction of
alkali metals changed the constants of the crystal
lattice of the K NiF type. On introducing the larger
2
4
2
4
EXPERIMENTAL
Powder catalysts were synthesized using La(NO )
cesium ion, these constants were 0.2% greater than
those upon substitution of lanthanum with potassium.
The activity of multicomponent compounds was eval-
uated in carbon oxidation with atmospheric oxygen,
with step-by-step isothermal (20 min) exposures of
the reaction mixtures in the temperature range from
100 to 600 C. As a reference sample served carbon
without a catalyst or with addition of indifferent sub-
stances of mass equal to that of the catalyst. The
3 3
6H O, CsNO , KNO , and AgNO (all of chemically
2
3
3
3
pure grade), NH VO of pure grade, Cu(NO ) 3H O,
4
3
3 2
2
4 y
and polyvinyl alcohol of 11/2 brand. Cs La VO
x
1
x
(x = 0 0.5) was synthesized by pyrolysis of polymer
salt formulations [13, 14]. A 5 10% solution of poly-
vinyl alcohol in distilled water was prepared on a
water bath and prescribed amounts of La(NO )
nominal reaction ignition temperature T was deter-
3 3
ig
6H O and CsNO were dissolved in this solution.
mined from the temperature dependences of the degree
of carbon burning-out. The activity of Cs La
2
3
Ammonium vanadate was dissolved in water under
heating, and the resulting solution was mixed with
a separate portion of the aqueous solution of polyvinyl
alcohol and with an aqueous-polymeric solution of the
nitrates. A gel prepared using the methods described
in [15, 16] was dried in a thin layer under an infrared
lamp and subjected to pyrolysis and then to calci-
nation (4 h at 600 C). The product obtained was
analyzed on a DRON-1.5 diffractometer with CoK
radiation using the JCPDS file. To introduce the
x
1 x
VO
M
exceeded (T
=
300 310 C) that of
. An analysis of the activ-
4 y
4
y
ig
La Cu
V
O
0.1 1.9 0.95 0.05
ity of catalysts of the last type confirmed its relation-
ship with the atomic mass of the alkali metal ion
incorporated into the structure. For the composition
+
+
containing Cs or K , the ignition temperatures were
330 and 340 C, respectively. For samples without
a catalyst, the ignition temperature was 410 C. It
was found that the optimal catalyst composition is
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OXIDATION OF CARBON IN THE PRESENCE OF CATALYSTS
1123
Table 1. Ignition temperature of carbon oxidation
Cs La VO , and just this composition was used
0.3 0.7
4 y
+
in further experiments. A higher content of Cs com-
monly gives no additional increase in the catalytic
activity. A heterogeneous catalyst [17, 19, 20] of the
Sam-
ple
Ignition temperature Tig, C
Sample composition
no.
air
NO
empirical formula Ag La MnO
360 C), which exhibits high activity in oxidation of
CO and organic compounds, was also inferior in ac-
(T
=
0.25 0.75
3
y
ig
1
2
3
Carbon
Carbon + catalyst
Carbon + catalyst +
410
300 310
500
380
tivity to Cs La VO
.
0.3 0.7
4 y
In studying the kinetics of carbon oxidation, iso-
thermal experiments were performed in the practically
important working temperature range of catalysts,
promoter, wt %:
1
5
10
350
290
270
480
440
360
4
5
300 400 C. As promoter for Cs La VO
served
0.3 0.7
4 y
Cs SO . The conversion was determined at exposures
2
4
ranging from 5 min to 1 1.5 h. At a relatively short
duration of an experiment, the time of heating of the
samples exerts a noticeable influence, and there is
an induction period of the reaction, associated with
bonaceous particles in gas flows, it is advisable to
bring them in contact with solid surfaces at a suf-
ficiently high velocity, e.g., with a special baffler for
disintegration of ensembles of particles. Further, to
determine the kinetics of carbon oxidation in more
detail, it would be appropriate to perform experiments
with monodisperse powders. The carbon combustion
observed could also be described using the equation
of a first-order reaction with respect to carbon
the sorption of O on the carbon surface. It was con-
2
firmed in the course of the study that addition of
Cs SO to the catalyst markedly raises the rate of
2
4
carbon oxidation with oxygen and lowers the ignition
temperature of the reaction (Table 1, Fig. 1). In car-
bon oxidation in the kinetic mode at the external sur-
faces of grains, the dependences of the conversion
on time at isothermal exposures must be described
by the equation of a collapsing sphere
ln[1/(1
)] = k .
(3)
A calculation of the rate constants of the combus-
tion reaction by Eqs. (1) and (3) gave similar results.
[1
(1
)
1/3] = k ,
(1)
In the presence of catalysts based on cesium lan-
thanum vanadate, the overall rate constant of carbon
oxidation, K, was higher (Table 2), compared with the
case of their absence. The values of E for carbon oxi-
dation in the presence of catalysts for sample nos. 2 5
are in good agreement with the data obtained by other
authors [12]. Consequently, it may be considered
that the oxidation mode is close to that of burning of
separate particles in a gas medium; the preexponential
factors also conform to this model.
where k is the rate constant.
On the whole, the experimental data can be de-
scribed, within the experimental error, by the above
equation, which is confirmed by data processing in the
coordinates of reduced time. However, deviations
from this dependence are observed in some cases at
relatively high conversions, with the conversion found
to be somewhat lower than the calculated value. This
may be due to existence in the final stage of residues
of the coarsest or difficultly oxidizable particles,
because the carbon powders used were not exactly
monodisperse and homogeneous: they were produced
from raw materials of natural origin. Moreover, a cer-
tain contribution may come from carbon particles with
nonspherical shape. Such particles are similar to stable
chains in carbon black. For formations of this kind,
the time dependence of the conversion is described by
the equation of a collapsing cylinder, which also
leads to a certain decrease in the rate of the overall
oxidation process:
Introduction of 1 wt % Cs SO dramatically
2
4
changes the properties of the catalyst: E and T
ig
increase (Tables 1, 2; Figs. 2, 3). Also strongly grows
the pre-exponential factor, with the result that the rate
of carbon oxidation, on the whole, increases. The
phenomenon observed can be attributed to changes in
the nature of the phases present in the catalyst and the
extent of contact with carbon particles at the boundary
of the catalyst. As the content of Cs SO in the
2
4
catalyst increases, the activation energy of combustion
becomes lower, as also does the ignition temperature.
In the presence of 10 wt % Cs SO (Table 2), E
[1
(1
)
1/2] = k .
(2)
2
4
has a value characteristic of burning in a bed.
For practical implementation of oxidation of car-
For noncatalytic carbon oxidation, the E obtained has
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 7 2004

1124
OSTROUSHKO et al.
(a)
C
C
(b)
C
C
(c)
C
C
t, s
t, s
Fig. 1. Examples of the dependences of the extent of carbon removal, C, on the process duration t in oxidation with (I) nitrogen
oxide and (II) atmospheric oxygen. Temperature ( C): (a) 300, (b) 350, and (c) 400. Sample: (1) without catalyst, (2) carbon
with catalyst without promoter, (3) carbon with catalyst containing 1 wt % promoter, (4) the same with 5 wt % promoter, and
(5) the same with 10 wt % promoter.
an intermediate value between the cases of particle
burning and carbon oxidation in a bed [12]. In this
occur at the internal surface of particles, which
require an internal diffusion of oxygen and back
transport of oxidation products, in accordance with
the equation derived by L.N. Khitrin [12]:
case, the values of K (Table 2) do not correspond
0
to those calculated using the particle burning
equations. Because the rate of noncatalytic oxidation
of carbon is, on the whole, lower, the combustion
parameters are strongly affected by reactions that
KSC
=
c0(1/K1 + R/D) ,
(4)
1
where c is the ambient oxygen concentration; D, the
0
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OXIDATION OF CARBON IN THE PRESENCE OF CATALYSTS
1125
Table 2. Kinetic parameters of catalytic oxidation of carbon with oxygen
1
Pre-exponential factor logK0 (K0, s )
Sample
no.
E,
kJ mol
Sample composition
1
experiment
1*
2*
3*
1
2
Carbon
Carbon + catalyst
55
70
0.52
1.93
1.80
2.42
1.69
2.05
3.10
3.40
Carbon + catalyst + promoter, wt %:
3
4
5
6
1
5
10
88
80
25
88
3.64
3.30
1.40
3.53
3.18
2.84
0.54
3.18
2.48
2.29
0.99
2.48
3.76
3.60
2.50
3.76
Carbon + Ag0.25La0.75MnO3
y
4
* Calculated by equations [16] suggested by (1) L.A. Vulis, logK = 1.75 10
E
0.5; (2) G.F. Knorre, I.I. Paleev, and
0
4
4
K.M. Aref’ev, logK = 0.991 10 E + 0.398; and (3) S.M. Shestakov, logK = 0.838 10 E + 2.
0
0
diffusion coefficient; K , the overall reaction rate con-
stant; and R, the particle radius.
1
C, %
Here, we should take into account the factor
,
which is the stoichiometric coefficient (mass ratio of
reacted carbon to spent oxygen) determined by rela-
tive yields of CO and CO in oxidation of carbon:
2
12
=
(1
),
(5)
32
where = 1 for yield of only CO, = 0 for yield of
CO , and = 0.33 at CO/CO = 1; 12 and 32 are the
2
2
molecular weights of carbon and oxygen.
The value of in the presence of catalysts is, prob-
ably, substantially higher. The catalysts can affect
both the reaction of carbon with atmospheric oxygen
[reaction (6)] and the oxidation of CO formed in the
process [reaction (7)]:
T, C
Fig. 2. Extent C of carbon removal at a 20-min exposure in
oxidation with (1, 2, 4) atmospheric oxygen and (3, 5 9) ni-
trogen oxide vs. temperature T. Sample: (1 [9], 3) carbon
without catalyst, (6) carbon with catalyst without promoter,
(7) carbon with catalyst containing 1 wt % promoter,
(8) the same with 5 wt % promoter, (4, 9) the same with
10 wt % promoter, and (2, 5) carbon with promoter.
C + 1/2O2
CO,
(6)
(7)
CO + 1/2O2
CO2.
1
Tig, C
E, kJ mol
Both the reactions are rather complex. Depending
on the influence exerted by a catalyst on processes (6)
and (7), the overall oxidation mechanism may also
vary. In the absence of a promoter or at its low con-
tent, it may be assumed, by virtue of the limited area
of contact between carbon and the catalyst, that CO
oxidation (7) predominates. This leads to a higher
value of factor . In the presence of a catalyst, reac-
tion (7) is, probably, localized at the surface of the
complex oxide. In this case, the effective activation
energy is comparable with the values of E for CO oxi-
m, wt %
Fig. 3. Nominal ignition temperature T and effective
ig
1
dation on complex oxide catalysts (64 to 132 kJ mol )
[17], which confirms the assumption made. Estimat-
ing the specific rate of oxidation at the catalyst surface
activation energy E vs. the amount of promoter introduced,
m, in carbon oxidation with (1) nitrogen oxide and
(2) atmospheric oxygen.
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1126
OSTROUSHKO et al.
of CO formed in the combustion of carbon gives
values that are close to the rate of catalytic CO oxida-
tion proper [17]. In oxidation of carbon in the pres-
ence of catalysts in the temperature interval 300
400 C, the amount of burning CO is within the range
envelop carbon particles, thereby increasing the extent
of contact. In this case, it becomes understandable
why the effective E for sample no. 5 (Table 2) strong-
ly decreases and the process passes into the diffusion-
controlled mode, in which the overall rate of carbon
oxidation is limited by the supply of the oxidizing
agent from air into the reaction zone or by removal of
gaseous products. Here, as also for sample no. 1, a
discrepancy is observed between the experimental
17
2
(1.3 6.1) 10 molecules/(m s). According to [17],
17
18
2
2.12 10 3.44 10 CO molecules/(m s) burns
at 240 C on complex oxide catalysts. This also con-
firms the hypothesis formulated here. The possible
elementary event determining the rate of the overall
process is interaction of adsorbed CO with dissocia-
tively adsorbed oxygen at the surface of the complex
oxide. Intermediate species of the carbonate type are
formed in the course of the reaction, and decomposi-
tion of these species is facilitated by transfer of an
electron from a cation of the d metal of the complex
oxide to the oxygen molecule. Thus, a local reduction
of the surface of the catalyst, which contains ions with
variable oxidation state, occurs in the course of the
reaction. Vanadium can change its oxidation state as
values of K and those calculated using the particle
0
combustion equations. It was shown in special-pur-
pose experiments (Fig. 2) that the promoter itself also
affects the carbon oxidation. Technical-grade Cs SO
2
4
shows an acid reaction and contains CsHSO impuri-
4
ties. It may be assumed that the oxidation reaction is
initiated by interaction with this impurity contained in
the promoter, e.g.:
4CsHSO4 + C
or
2H2O + CO2 + 2Cs2SO4 + 2SO2, (8)
5+
4+
follows: V
V . In addition, an intrinsic non-
2CsHSO4 + C
2CsHSO4 + C
Cs2CO3 + H2O + 2SO2,
(9)
stoichiometry may exist in the compound Cs La
x
1 x
VO , and, therefore, oxygen incorporated into the
4
y
Cs2SO4 + H2O + SO2 + CO. (10)
structure of the vanadate may be partly involved in the
reactions at elevated temperatures. It can be seen from
the previously obtained gravimetric curves [21] that
reversible changes in the mass of complex oxide sam-
ples occur at the working temperatures in the presence
of a carbon residue. After local reduction, the catalyst
surface is again oxidized with oxygen, and the next
cycle of CO oxidation occurs. However, the described
factor affecting the oxidation is hardly the only factor,
because catalysts based on manganites (Table 2) and
cuprates are somewhat inferior to vanadate catalysts in
their activity in oxidation of carbon, although the
bonding energy of oxygen, which commonly deter-
mines the catalytic activity in CO oxidation [22, 23],
is substantially lower than that in the vanadium
compounds.
Reactions of this kind give rise to carbon burning,
and they are complete after the acid part of the pro-
moter is exhausted and no considerable amounts of
SO are evolved. A thermodynamic estimation of
2
reactions of this type, with compounds of alkali met-
als involved, demonstrated that they can be performed
at low temperatures. The probability of occurrence of
reaction (8) is the highest, and that of reaction (10),
the lowest. Some promoter samples contained admix-
tures of CsNO , which has a melting point of 417 C
3
and can act as a carbon-oxidizing agent, especially
in a melt. However, when a promoter containing no
impurities of the above kind was used, the type of
its influence on the carbon oxidation remained un-
changed. It should be assumed in this case that the
promoter has a catalytic effect, which is determined
by the fact that Cs SO is an alkali metal salt. As
The aforesaid suggests that there should exist a
direct influence of the catalyst on the primary oxida-
tion reaction (6) in the zone of contact with carbon,
which may be particularly strongly manifested in the
presence of a considerable amount of a promoter. This
is confirmed by the fact that the catalytic activity of
the compounds under study in the given reaction
depends on the atomic mass of the dopant cation. At
places of contact between the catalyst and carbon, the
alkali metal ion can form surface salt complexes of
the carbonate type [8]. In addition, raising the amount
of promoter makes lower the melting point of the
catalytic formulation, and it gains the capacity to
2
4
already mentioned, alkali metal compounds exhibit a
catalytic activity in heterogeneous reactions with
carbon [8]. It may be stated that cesium lanthanum
vanadate and, if present in a considerable amount,
Cs SO are cocatalysts.
2
4
The carbon oxidation with nitrogen oxide NO is
also accelerated in the presence of a catalyst. At rela-
tively low temperatures (Figs. 1, 2), the rates of oxi-
dation with nitrogen oxides and atmospheric oxygen
are comparable or the rate of the former process is
somewhat higher. On raising the temperature, the oxi-
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OXIDATION OF CARBON IN THE PRESENCE OF CATALYSTS
1127
b
Table 3. Exponent b in the kinetic equation k = a
for carbon oxidation with nitrogen oxide
b at indicated temperature,
C
Sample no.
Sample composition
300
350
400
1
2
Carbon
Carbon+ catalyst
0.50
0.37
0.36
0.36
0.20
0.45
Carbon + catalyst +promoter, wt %:
3
4
5
1
5
10
0.34
0.43
0.24
0.39
0.41
0.23
0.33
0.69
0.44
dation with nitrogen oxides is slower than that with
sing of experimental data by Eqs. (1) (3) failed to
produce any satisfactory result. It was established
using the Table Curve 2D v2.0 program by AISN
Software Inc. that the experimental data are better
described by power-law equations. This software
made it possible to find the exponent b in an equation
O . The complete oxidation of carbon includes two
2
main stages:
C + NO
CO + 1/2N2,
CO2 + 1/2N2.
(11)
(12)
b
CO + NO
of the type k = a (Table 3). The most pronounced
change in the reaction mechanism is observed for
noncatalytic oxidation and for catalysts containing 5
and 10 wt % Cs SO . The trends exhibited by the
exponent b for different samples are the opposite. For
sample no. 1, E increases as the temperature is raised,
and for sample nos. 4 and 5, contrariwise, decreases.
There is published evidence that the oxidizing
capacity of nitrogen oxides NO [23] and N O [9]
2
2
4
toward CO is lower than that of O . This is accounted
2
for, in particular, by the strong difference between
the sorption properties of nitrogen oxides and O on
2
the surface of complex oxide catalysts. A possible
rate-determining stage of the overall process of carbon
oxidation with nitrogen oxide is reaction (12), which
is localized at the surface of the complex oxide. To
verify this assumption, it is important to study the in-
fluence exerted by the promoter on the interaction of
carbon with NO. Addition of Cs SO plays a less
These facts may point to an oxidation mechanism in
which direct chemical interaction is not the key factor.
It is known [9] that carbon oxidation with nitrogen
oxide NO can be hindered at relatively low tempera-
tures by formation of N O, whose catalytic decom-
2
position rate is low. In the temperature range studied,
2
4
formation of N O is hardly possible. This is also in-
positive role in reactions with NO, compared with the
effect of the promoter Cs SO on the carbon oxidation
2
dicated by effective values of E for samples with the
weakest temperature dependence of the exponent b.
These values are markedly lower than those for the
2
4
with oxygen (Figs. 1, 2). This, probably, occurs
because of the change in the sorption capacity of
the catalyst surface in the presence of Cs SO . To
1
carbon oxidation with O and N O (146 kJ mol ) [9],
2
2
2
4
1
evaluate the nominal ignition temperatures, experi-
ments were performed with catalyst carbon mixtures
kept at prescribed temperatures for 20 min (Table 1;
Figs. 2, 3). As also in the case of carbon oxidation
with O , introduction of 1 wt % Cs SO markedly
being equal to 6 20 kJ mol , which is characteristic
of sorption or diffusion processes. The decrease in the
activation energy for the reaction of carbon oxidation
with nitrogen oxide NO, compared to that with at-
mospheric oxygen, can be attributed to differences
2
2
4
changes the properties of the catalyst.
between the heats of O and NO sorption on the cata-
2
lyst surface. Moreover, the oxidation of carbon with
nitrogen oxide is more thermodynamically feasible
than that with oxygen: the enthalpy of formation of
nitrogen oxides is positive. The low effective activa-
tion energy of carbon oxidation with nitrogen oxide
results in that the oxidation can occur in this case at
lower temperatures, compared to the reaction in air
(Fig. 2), with only stage (11) probable. At the same
time, oxidation of CO with nitrogen oxide NO [reac-
In the temperature range studied, it was more dif-
ficult to estimate the effective activation energy of
carbon oxidation with nitrogen oxide NO, compared
to oxidation with atmospheric oxygen, because the
activation energy is temperature-dependent. This
points to a significant change in the reaction mechan-
ism with increasing temperature. It is known for
carbon that the mechanism of formation of a surface
oxide changes in the range 300 550 C [18]. Proces-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 7 2004

1128
OSTROUSHKO et al.
tion (12)] occurs at the catalyst surface. The rate of
reaction (12) is considerably lower than that of the
CO oxidation with oxygen [reaction (7)], even in the
presence of catalysts.
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